By Lucy Craymer

WELLINGTON (Reuters) -New Zealand police said on Wednesday a fire at a hostel in the capital, Wellington, in which at least six people were killed was suspected to have been arson and they had opened a homicide inquiry.

A blaze broke out on the top floor of the Loafers Lodge in the suburb of Newtown in the early hours of Tuesday, causing major structural damage that is hampering recovery efforts.

Following a health and safety assessment, police said investigators hoped to enter the 92-room building in the afternoon to begin their investigation, and find and identify the dead.

"This scene examination will be an extensive and methodical process, and we expect it to take some time – likely several days," Acting Wellington District Commander Dion Bennett said.

"I can confirm that we are treating the fire as arson."

The hostel provided accommodation for construction workers, hospital staff and those serving sentences in the community for minor crimes, among other people.

Police said on Wednesday that some people were missing, putting the number at less than 20 people. They said some of the missing residents of the hostel might just be unaccounted for.

The cause of the fire was not known but police said there was a couch fire late on Monday, two hours before the fatal fire, but it had not been reported to emergency services at the time.

"We will be seeking to confirm any link between that couch fire and the subsequent fatal fire," Bennett said.

Earlier on Wednesday, Fire and Emergency New Zealand (FENZ) deemed it safe for police to enter the building.

"The next step for us is going through and ensuring that there isn't anybody else in there and obviously working with police to investigate and support them in the removal of the six people," FENZ region manager Bruce Stubbs told Radio New Zealand.

(Reporting by Lucy Craymer in Wellington;Editing by Matthew Lewis, Robert Birsel)